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More Efficient Solar Cells

A semiconductor material with three energy bands uses more sunlight, by trapping low-energy photons.

Researchers at Lawrence Berkeley National Laboratory (LBNL) have created a new type of semiconductor material designed to improve the efficiency of solar cells by capturing low-energy photons.

Traditional solar cells respond only to a narrow spectrum of sunlight, making them highly inefficient. In the language of physicists, solar cells convert light with wavelengths corresponding to the energy it takes for electrons to jump from the valence band to the conduction band. Photons with lower energy pass right through the material.

The new semiconductor material can capture these low-energy photons for electricity, which could make solar cells with efficiencies of around 45 percent, compared with 25 percent for conventional cells that use a single semiconductor and 39 percent for cells with layers of mixed semiconductors.

The new semiconductors have three energy bands instead of the usual two (valence and conduction). The third band lies below the conduction band, effectively splitting the gap between the valence and conduction bands into two smaller parts. “This helps low-energy photons to participate in the process because they can excite [electrons] to the [intermediate] band and then up. It’s like a stepping stone,” says Wladek Walukiewicz of LBNL’s Materials Sciences Division, who developed the semiconductor with colleague Kin Man Yu.

The researchers found that introducing a few atoms of oxygen into a zinc-manganese-tellurium (ZnMnTe) alloy splits the compound semiconductor’s conduction band into two parts. Similarly, adding nitrogen to a semiconductor such as gallium arsenide phosphide will also give a multi-band semiconductor.

LBNL has licensed the technology to RoseStreet Labs, a startup in Phoenix, AZ, which plans to commercialize solar cells made from these multi-band semiconductors. Because it’s an entirely new technology, though, it’s hard to say when such a solar cell will be available, Walukiewicz says.

Existing solar cells with the best efficiencies–those as high as 39 percent–convert light into electricity by using different semiconductor materials with different band gaps, which are stacked on top of each other to capture a broader spectrum of light wavelengths. But these solar cells are expensive, limiting their application to uses in satellites. A device made from a single, multi-band semiconductor would likely be cheaper and easier to make, says Walukiewicz.

Nonetheless, adding oxygen to the ZnMnTe alloy is hard, because oxygen does not mix readily with tellurium. To make the new materials, then, the researchers have developed a method that implants highly energetic oxygen atoms into the alloy using an ion beam. Then they use “a very short pulse of laser to melt the material and rapidly regrow it so that the oxygen is all trapped inside,” says Yu.

Making a solar cell from gallium arsenide phosphide should be easier, the researchers say, because gallium arsenide compounds can be grown layer by layer.

To reach 40 percent efficiency, though, the semiconductor material and solar cell will have to meet some fundamental requirements of physics, says Sarah Kurtz, a senior scientist at the National Renewable Energy Laboratory in Golden, CO. For instance, efficiency goes down if the material has defects or if the light isn’t absorbed as the designers intended. But, says Kurtz, if the LBNL researchers are able to overcome these challenges, “this would represent a breakthrough.”

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